Asymmetric or symmetric? Cytosolic modulation of human erythrocyte hexose transfer

Michaelis-Menten kinetic modeling of hemoglobin A1c status facilitates personalized glycemic control

INTRODUCTION

Discrepancy between measured HbA1c and HbA1c calculated from plasma glucose is associated with higher risk for diabetic complications. However, quantification of this difference is inaccurate due to the imperfect linear conversion models. We propose to introduce a mathematical formula that correlates with the observational data and supports individualized glycemic control.

METHODS

We analysed 175,437 simultaneous plasma glucose and HbA1c records stored in our laboratory database. Employing the Michaelis-Menten (MM) equation, we compared the calculated HbA1c levels to the measured HbA1c levels. Data from patients with multiple records were used to establish the patients' glycemic status and to assess the predictive power of our MM model.

RESULTS

HbA1c levels calculated with the MM equation closely matched the population's average HbA1c levels. The Michaelis constant (Km) had a negative correlation with HbA1c (r2 = 0.403). Using personalized Km values in the MM equation, 85.1% of HbA1c predictions were within 20% error (ADAG calculation: 78.4%). MM prediction also performed better in predicting pathologic HbA1c levels (0.904 AUC vs. 0.849 AUC for ADAG).

CONCLUSION

MM equation is an improvement over linear models and could be readily employed in routine diabetes management. Km is a reliable and quantifiable marker to characterize variations in glucose tolerance.

Erythroid glucose transport in health and disease

Glucose transport is intimately linked to red blood cell physiology. Glucose is the unique energy source for these cells, and defects in glucose metabolism or transport activity are associated with impaired red blood cell morphology and deformability leading to reduced lifespan. In vertebrate erythrocytes, glucose transport is mediated by GLUT1 (in humans) or GLUT4 transporters. These proteins also account for dehydroascorbic acid (DHA) transport through erythrocyte membrane. The peculiarities of glucose transporters and the red blood cell pathologies involving GLUT1 are summarized in the present review.

Human erythrocytes transport dehydroascorbic acid and sugars using the same transporter complex

GLUT1, the primary glucose transport protein in human erythrocytes [red blood cells (RBCs)], also transports oxidized vitamin C [dehydroascorbic acid (DHA)]. A recent study suggests that RBC GLUT1 transports DHA as its primary substrate and that only a subpopulation of GLUT1 transports sugars. This conclusion is based on measurements of cellular glucose and DHA equilibrium spaces, rather than steady-state transport rates. We have characterized RBC transport of DHA and 3-O-methylglucose (3-OMG), a transported, nonmetabolizable sugar. Steady-state 3-OMG and DHA uptake in the absence of intracellular substrate are characterized by similar Vmax (0.16 ± 0.01 and 0.13 ± 0.02 mmol·l(-1)·min(-1), respectively) and apparent Km (1.4 ± 0.2 and 1.6 ± 0.7 mM, respectively). 3-OMG and DHA compete for uptake, with Ki(app) of 0.7 ± 0.4 and 1.1 ± 0.1 mM, respectively. Uptake measurements using RBC inside-out-membrane vesicles demonstrate that 3-OMG and DHA compete at the cytoplasmic surface of the membrane, with Ki(app) of 0.7 ± 0.1 and 0.6 ± 0.1 mM, respectively. Intracellular 3-OMG stimulates unidirectional uptake of 3-OMG and DHA. These findings indicate that DHA and 3-OMG bind at mutually exclusive sites at exo- and endofacial surfaces of GLUT1 and are transported via the same GLUT1 complex.

Will the original glucose transporter isoform please stand up!

Monosaccharides enter cells by slow translipid bilayer diffusion by rapid, protein-mediated, cation-dependent cotransport and by rapid, protein-mediated equilibrative transport. This review addresses protein-mediated, equilibrative glucose transport catalyzed by GLUT1, the first equilibrative glucose transporter to be identified, purified, and cloned. GLUT1 is a polytopic, membrane-spanning protein that is one of 13 members of the human equilibrative glucose transport protein family. We review GLUT1 catalytic and ligand-binding properties and interpret these behaviors in the context of several putative mechanisms for protein-mediated transport. We conclude that no single model satisfactorily explains GLUT1 behavior. We then review GLUT1 topology, subunit architecture, and oligomeric structure and examine a new model for sugar transport that combines structural and kinetic analyses to satisfactorily reproduce GLUT1 behavior in human erythrocytes. We next review GLUT1 cell biology and the transcriptional and posttranscriptional regulation of GLUT1 expression in the context of development and in response to glucose perturbations and hypoxia in blood-tissue barriers. Emphasis is placed on transgenic GLUT1 overexpression and null mutant model systems, the latter serving as surrogates for the human GLUT1 deficiency syndrome. Finally, we review the role of GLUT1 in the absence or deficiency of a related isoform, GLUT3, toward establishing the physiological significance of coordination between these two isoforms.

alpha- and beta-monosaccharide transport in human erythrocytes

Equilibrative sugar uptake in human erythrocytes is characterized by a rapid phase, which equilibrates 66% of the cell water, and by a slow phase, which equilibrates 33% of the cell water. This behavior has been attributed to the preferential transport of beta-sugars by erythrocytes (Leitch JM, Carruthers A. Am J Physiol Cell Physiol 292: C974-C986, 2007). The present study tests this hypothesis. The anomer theory requires that the relative compartment sizes of rapid and slow transport phases are determined by the proportions of beta- and alpha-sugar in aqueous solution. This is observed with D-glucose and 3-O-methylglucose but not with 2-deoxy-D-glucose and D-mannose. The anomer hypothesis predicts that the slow transport phase, which represents alpha-sugar transport, is eliminated when anomerization is accelerated to generate the more rapidly transported beta-sugar. Exogenous, intracellular mutarotase accelerates anomerization but has no effect on transport. The anomer hypothesis requires that transport inhibitors inhibit rapid and slow transport phases equally. This is observed with the endofacial site inhibitor cytochalasin B but not with the exofacial site inhibitors maltose or phloretin, which inhibit only the rapid phase. Direct measurement of alpha- and beta-sugar uptake demonstrates that erythrocytes transport alpha- and beta-sugars with equal avidity. These findings refute the hypothesis that erythrocytes preferentially transport beta-sugars. We demonstrate that biphasic 3-O-methylglucose equilibrium exchange kinetics refute the simple carrier hypothesis for protein-mediated sugar transport but are compatible with a fixed-site transport mechanism regulated by intracellular ATP and cell shape.

Alternating carrier models of asymmetric glucose transport violate the energy conservation laws

Alternating access transporters with high-affinity externally facing sites and low-affinity internal sites relate substrate transit directly to the unliganded asymmetric "carrier" (Ci) distribution. When both bathing solutions contain equimolar concentrations of ligand, zero net flow of the substrate-carrier complex requires a higher proportion of unliganded low-affinity inside sites (proportional, variant 1/KD(in)) and slower unliganded "free" carrier transit from inside to outside than in the reverse direction. However, asymmetric rates of unliganded carrier movement, kij, imply that an energy source, DeltaGcarrier = RT ln (koi/kio) = RT ln (Cin/Cout) = RT ln (KD(in)/KD(out)), where R is the universal gas constant (8.314 Joules/M/K degrees), and T is the temperature, assumed here to be 300 K degrees , sustains the asymmetry. Without this invalid assumption, the constraints of carrier path cyclicity, combined with asymmetric ligand affinities and equimolarity at equilibrium, are irreconcilable, and any passive asymmetric uniporter or cotransporter model system, e.g., Na-glucose cotransporters, espousing this fundamental error is untenable. With glucose transport via GLUT1, the higher maximal rate and Km of net ligand exit compared to net ligand entry is only properly simulated if ligand transit occurs by serial dissociation-association reactions between external high-affinity and internal low-affinity immobile sites. Faster intersite transit rates occur from lower-affinity sites than from higher-affinity sites and require no other energy source to maintain equilibrium. Similar constraints must apply to cotransport.

Supply and demand in cerebral energy metabolism: the role of nutrient transporters

Glucose is the obligate energetic fuel for the mammalian brain, and most studies of cerebral energy metabolism assume that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state. Glucose transporter (GLUT) proteins deliver glucose from the circulation to the brain: GLUT1 in the microvascular endothelial cells of the blood-brain barrier (BBB) and glia; GLUT3 in neurons. Lactate, the glycolytic product of glucose metabolism, is transported into and out of neural cells by the monocarboxylate transporters (MCT): MCT1 in the BBB and astrocytes and MCT2 in neurons. The proposal of the astrocyte-neuron lactate shuttle hypothesis suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energetics, especially during activation. Since the identification of the GLUTs and MCTs in brain, much has been learned about their transport properties, that is capacity and affinity for substrate, which must be considered in any model of cerebral glucose uptake and utilization. Using concentrations and kinetic parameters of GLUT1 and -3 in BBB endothelial cells, astrocytes, and neurons, along with the corresponding kinetic properties of the MCTs, we have successfully modeled brain glucose and lactate levels as well as lactate transients in response to neuronal stimulation. Simulations based on these parameters suggest that glucose readily diffuses through the basal lamina and interstitium to neurons, which are primarily responsible for glucose uptake, metabolism, and the generation of the lactate transients observed on neuronal activation.

ATP-dependent sugar transport complexity in human erythrocytes

Human erythrocyte glucose sugar transport was examined in resealed red cell ghosts under equilibrium exchange conditions ([sugar](intracellular) = [sugar](extracellular), where brackets indicate concentration). Exchange 3-O-methylglucose (3MG) import and export are monophasic in the absence of cytoplasmic ATP but are biphasic when ATP is present. Biphasic exchange is observed as the rapid filling of a large compartment (66% cell volume) followed by the slow filling of the remaining cytoplasmic space. Biphasic exchange at 20 mM 3MG eliminates the possibility that the rapid exchange phase represents ATP-dependent 3MG binding to the glucose transport protein (GLUT1; cellular [GLUT1] of </=20 microM). Immunofluorescence-activated cell sorting analysis shows that biphasic exchange does not result from heterogeneity in cell size or GLUT1 content. Nucleoside transporter-mediated uridine exchange proceeds as rapidly as 3MG exchange but is monoexponential regardless of cytoplasmic [ATP]. This eliminates cellular heterogeneity or an ATP-dependent, nonspecific intracellular diffusion barrier as causes of biphasic exchange. Red cell ghost 3MG and uridine equilibrium volumes (130 fl) are unaffected by ATP. GLUT1 intrinsic activity is unchanged during rapid and slow phases of 3MG exchange. Two models for biphasic sugar transport are presented in which 3MG must overcome a sugar-specific, physical (diffusional), or chemical (isomerization) barrier to equilibrate with cell water. Partial transport inhibition with the use of cytochalasin B or maltose depresses both rapid and slow phases of transport, thereby eliminating the physical barrier hypothesis. We propose that biphasic 3MG transport results from ATP-dependent, differential transport of 3MG anomers in which V(max)/apparent K(m) for beta-3MG exchange transport is 19-fold greater than V(max)/apparent K(m) for alpha-3MG transport.

A rapid method of reconstituting human erythrocyte sugar transport proteins

A rapid reconstitution procedure for human erythrocyte hexose transfer activity is described. The procedure (reverse-phase evaporation) avoids exposure of the isolated proteins to detergent, organic solvent, sonication, or freeze-thaw steps during insertion into synthetic membranes and may be effected within 15 min. The so-formed vesicles are unilamellar structures with a large encapsulated volume, narrow size range, and low passive permeabilities. Contamination by carry-through of endogenous (red cell) lipids is less than 1%. Reconstituted hexose transfer activity was examined by using unfractionated proteins (bands 3, 4.5, and 6) and purified proteins (bands 4.5 and 3). With unfractionated proteins, hexose transport activity is low [0.34 mumol X (mg of protein)-1 X min-1], is inhibited by cytochalasin B, and increases monotonically with protein concentration. Kinetic analysis indicates that Vmax values for both influx and efflux of D-glucose are identical. Reconstitution of the cytochalasin B binding protein (band 4.5) results in hexose transport with high specific activity [5 mumol X (mg of protein)-1 X min-1] and symmetry in transfer kinetics. Band 3 proteins also appear to mediate cytochalasin B sensitive D-glucose transport activity.

The effect of ATP-depletion on the inhibition of glucose exits from human red cells

The effect of ATP-depletion or its consequence, by metabolic inhibition, on the inhibition of glucose transport by various inhibitors was studied in human red cells. In cells depleted of ATP, glucose exit times were longer than in normal cells and the times increased with the duration of depletion. The Km for external glucose was higher in ATP-depleted cells than in normal undepleted cells (3.0 mM c.f. 2.5 mM at 30 degrees C). In contrast, the apparent Ki for cytochalasin B decreased from 0.85 microM in the normal cells to 0.5 microM after ATP-depletion. Half-maximal rates of glucose exit in the absence, and in the presence of 2 microM cytochalasin B were found at ATP concentrations of 0.43 and 0.68 microM, respectively. Although glucose exits from ATP-depleted cells exposed to the irreversible inhibitor of glucose transport, 1-fluoro-2,4-dinitrobenzene (FDNB) were slower than in normal cells, the relative degrees of inhibition were not significantly different. However, normal and ATP-depleted cells responded differently to treatment with 1,2-cyclohexanedione, a modifier of arginine residues which inhibits glucose exit. While normal cells were markedly inhibited, depleted cells were much less affected and the inhibitory effect of cytochalasin B seen in normal cells was reduced. These findings demonstrate that the glucose transport system of human red cells is affected by intracellular ATP and that ATP alters the affinity of the transporter for certain inhibitors. The implications of these findings are discussed.

Evidence from studies of temperature-dependent changes of D-glucose, D-mannose and L-sorbose permeability that different states of activation of the human erythrocyte hexose transporter exist for good and bad substrates

(1) The inhibition constant of L-sorbose flux from fresh human erythrocytes by D-glucose, Ki(sorbose) increases on cooling from 50 degrees C to 30 degrees C from 5.15 +/- 0.89 mM to 12.24 +/- 1.9 mM; the Ki(sorbose) of D-mannose increases similarly, indicating that the process is endothermic. (2) The activation energy Ea(sorbose) of net L-sorbose exit is 62.9 +/- 3.1 kJ/mol; in the co-presence of 5 mM D-glucose Ea(sorbose) is reduced to 41.7 +/- 1.6 kJ/mol (P < 0.005). (3) Cooling from 35 degrees C to 21 degrees C decreases the Ki(inf, cis) of auto-inhibition of D-glucose net exit from 5.2 +/- 0.3 mM to 1.36 +/- 0.06 mM; the Ki(inf, cis) of D-mannose falls from 10.9 +/- 1.65 mM to 5.7 +/- 0.3 mM. (4) The activation energy of D-glucose zero-trans net exit is 34.7 +/- 2.1 kJ/mol and that of D-mannose exit is 69.4 +/- 3.7 kJ/mol (P < 0.0025). (5) The exothermic and exergonic processes of auto-inhibition of D-glucose net exit are larger than those for D-mannose (P < 0.03). These data are consistent with D-glucose binding promoting an activated transporter state which following dissociation transiently remains; if an L-sorbose molecule binds within the relaxation time after D-glucose dissociation, it will have a higher mobility than otherwise. Cooling slows the relaxation time of the activated state hence raises the probability that L-sorbose will bind to the glucose-activated transporter. D-Glucose donates twice as much energy to the transporter as D-mannose, consequently produces more facilitation of flux. This view is inconsistent with the alternating carrier model of sugar transport in which net flux is considered to be rate-limited by return of the empty carrier, but is consistent with fixed two-site models.

Regulation of GLUT1-mediated sugar transport by an antiport/uniport switch mechanism

Avian erythrocyte sugar transport is stimulated during anoxia and during exposure to inhibitors of oxidative phosphorylation. This stimulation results from catalytic desuppression of the cell surface glucose transporter GLUT1 [Diamond, D., & Carruthers, A. (1993) J. Biol. Chem. 268, 6437-6444]. The present study was undertaken to investigate the mechanisms of GLUT1 suppression/desuppression. Sugar uniport (sugar uptake or exit in the absence of sugar at the opposite side of the membrane) is absent in normoxic avian erythrocytes, but sugar antiport (sugar uptake coupled to sugar exit) is present. Exposure to cyanide and/or to FCCP (mitochondrial inhibitors) stimulates erythrocyte sugar uniport but not sugar antiport. K(m)(app) for 3-O-methylglucose uniport and antiport are unaffected by metabolic poisoning. Ki(app) for inhibitions of 3-O-methylglucose uniport by cytochalasin B and forskolin (sugar export site ligands) are unaffected by progressive stimulation of sugar uniport. Cyanide and FCCP stimulation of 3-O-methylglucose uniport are associated with increased AMP-activated protein kinase activity. Purified human GLUT1 is not phosphorylated by exposure to cytosol extracted from poisoned avian erythrocytes. FCCP does not stimulate GLUT1-mediated 3-O-methylglucose uptake in K562 cells but does increase K562 AMP-activated protein kinase activity. FCCP stimulation of 3-O-methylglucose uniport in resealed erythrocyte ghosts requires cytosolic ATP and/or glutathione. The nonmetabolizable ATP analog AMP-PNP cannot be substituted for ATP in this action. These results are contrasted with allosteric regulation of human erythrocyte sugar transport and suggest that avian erythrocyte sugar transport suppression results from inhibition of carrier uniport function. Uniport suppression is not mediated by interaction with cytosolic molecular species that bind to the sugar export site. The antiport to uniport switch mechanism requires ATP hydrolysis, is associated with elevated AMP-activated kinase function, and, if triggered by this kinase, is mediated by factors absent in K562 cells and downstream from the kinase.

Human erythrocyte sugar transport is incompatible with available carrier models

GLUT1-mediated, passive D-glucose transport in human erythrocytes is asymmetric, Vmax and K(m)(app) for D-glucose uptake at 4 degrees C are 10-fold lower than Vmax and K(m)(app) for D-glucose export. Transport asymmetry is not observed for GLUT1-mediated 3-O-methylglucose transport in rat, rabbit, and avian erythrocytes and rat adipocytes where Vmax for sugar uptake and exit are identical. This suggests that transport asymmetry is either an intrinsic catalytic property of human GLUT1 or that factors present in human erythrocytes affect GLUT1-mediated sugar transport. In the present study we assess human erythrocyte sugar transport asymmetry by direct measurement of sugar transport rates and by analysis of the effects of intra- and extracellular sugars on cytochalasin B binding to the sugar export site. We also perform internal consistency tests to determine whether the measured, steady-state 3-O-methylglucose transport properties of human erythrocytes agree with those expected of two hypothetical models for protein-mediated sugar transport. The simple-carrier hypothesis describes a transporter that alternately exposes sugar import and sugar export pathways. The fixed-site carrier hypothesis describes a sugar transporter that simultaneously exposes sugar import and sugar export pathways. Steady-state 3-O-methylglucose transport in human erythrocytes at 4 degrees C is asymmetric. Vmax and K(m)(app) for sugar uptake are 10-fold lower than Vmax and K(m)(app) for sugar export. Phloretin-inhibitable cytochalasin B binding to intact red cells is unaffected by extracellular D-glucose but is competitively inhibited by intracellular D-glucose. This inhibition is reduced by 13% +/- 4% when saturating extracellular D-glucose levels are also present. Assuming transport is mediated by a simple-carrier and that cytochalasin B and intracellular D-glucose binding sites are mutually exclusive, the cytochalasin B binding data are explained only if transport is almost symmetric (Vmax exit = 1.4 Vmax entry). The cytochalasin B binding data are consistent with both symmetric and asymmetric fixed-site carriers. Analysis of 3-O-methylglucose, 2-deoxy-D-glucose, and D-glucose uptake in the presence of intracellular 3-O-methylglucose, demonstrates significant divergence in experimental and theoretical transport behaviors. We conclude either that human erythrocyte sugar transport is mediated by a carrier mechanism that is fundamentally different from those considered previously or that human erythrocyte-specific factors prevent accurate determination of GLUT1-mediated sugar translocation across the cell membrane. We suggest that GLUT1-mediated sugar transport in all cells is an intrinsically symmetric process but that intracellular sugar complexation in human red cells prevents accurate determination of transport rates.

Insulinomimetic effects of myricetin on lipogenesis and glucose transport in rat adipocytes but not glucose transport translocation

Myricetin is a naturally occurring flavonol that is commonly found in tea, berries, fruits, and medicinal plants. It mimics insulin in stimulating lipogenesis and glucose transport in rat adipocytes in vitro. It was found to stimulate lipogenesis in rat adipocytes and enhance the stimulatory effect of insulin. The EC50 was estimated to be about 65 microM. Myricetin did not have any effect on insulin receptor autophosphorylation nor on the tyrosine kinase activity of the receptor. However, myricetin stimulated both D-glucose and D-3-O-methylglucose uptake in rat adipocytes. The Vmax of glucose transport was increased, but the Km did not change significantly. Immunoblot analysis of Glut4 in rat adipocyte plasma membrane showed that the stimulation of glucose transport was not a consequence of glucose transporter translocation. Instead, the stimulation in glucose uptake probably was due to a change in the intrinsic activity of the glucose transporter possibly caused by alterations in membrane fluidity or transporter-lipid interactions as a result of the insertion of myricetin into the membrane bilayer. Thus, myricetin may have therapeutic potential in the management of non-insulin-dependent diabetes mellitus by stimulating glucose uptake without the presence of fully functional insulin receptor.

Glucose transporter function is controlled by transporter oligomeric structure. A single, intramolecular disulfide promotes GLUT1 tetramerization

The human erythrocyte glucose transporter is an allosteric complex of four GLUT1 proteins whose structure and substrate binding properties are stabilized by reductant-sensitive, noncovalent subunit interactions [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838]. In the present study, we use biochemical and molecular approaches to isolate specific determinants of transporter oligomeric structure and transport function. When unfolded in denaturant, each subunit (GLUT1 protein) of the transporter complex exposes two sulfhydryl groups. Four additional thiol groups are accessible following subunit exposure to reductant. Assays of subunit disulfide bridge content suggest that two inaccessible sulfhydryl groups form an internal disulfide bridge. Differential alkylation/peptide mapping/N-terminal sequence analyses show that a GLUT1 carboxyl-terminal peptide (residues 232-492) contains three inaccessible sulfhydryl groups and that an N-terminal GLUT1 peptide (residues 147-261/299) contains two accessible thiols. The carboxyl-terminal peptide most likely contains the intramolecular disulfide bridge since neither its yield nor its electrophoretic mobility is altered by addition of reductant. Each GLUT1 cysteine was changed to serine by oligonucleotide-directed, in vitro mutagenesis. The resulting transport proteins were expressed in CHO cells and screened by immunofluorescence microscopy for their ability to expose tetrameric GLUT1-specific epitopes. Serine substitution at cysteine residues 133, 201, 207, and 429 does not inhibit exposure of tetrameric GLUT1-specific epitopes. Serine substitution at cysteines 347 or 421 prevents exposure of tetrameric GLUT1-specific epitopes. Hydrodynamic analysis of GLUT1/GLUT4 chimeras expressed in and subsequently solubilized from CHO cells indicates that GLUT1 residues 1-199 promote chimera dimerization and permit GLUT1/chimera heterotetramerization. This GLUT1 N-terminal domain is insufficient for chimera tetramerization which additionally requires GLUT1 residues 200-463. Extracellular reductants (dithiothreitol, beta-mercaptoethanol, or glutathione) reduce erythrocyte 3-O-methylglucose uptake by up to 15-fold. This noncompetitive inhibition of sugar uptake is reversed by the cell-impermeant, oxidized glutathione. Reductant is without effect on sugar exit from erythrocytes. Dithiothreitol doubles the cytochalasin B binding capacity of erythrocyte-resident glucose transporter, abolishes allosteric interactions between substrate binding sites on adjacent subunits, and occludes tetrameric GLUT1-specific GLUT1 epitopes in situ. CHO cell-resident GLUT1 structure and transport function are similarly affected by extracellular reductant. We conclude that each subunit of the glucose transporter contains an extracellular disulfide bridge (Cys347 and Cys421) that stabilizes transporter oligomeric structure and thereby accelerates transport function.

Rapid substrate translocation by the multisubunit, erythroid glucose transporter requires subunit associations but not cooperative ligand binding

The human erythroid glucose transporter is a GLUT1 homotetramer whose structure and function are stabilized by noncovalent, cooperative subunit interactions. The present study demonstrates that exofacial tryptic digestion of GLUT1 abolishes cooperative interactions between substrate binding sites on adjacent subunits under circumstances where subunit associations and high catalytic turnover are maintained. Extracellular trypsin produces rapid, quantitative cleavage of the human red cell-resident sugar transport protein, GLUT1. One major carboxyl-terminal peptide of M(r)(app) 25,000 is detected by immunoblot analysis. Endofacial tryptic digestion of GLUT1 results in the complete loss of GLUT1 carboxyl-terminal structure. GLUT1-mediated erythrocyte sugar uptake, transport inhibition by cytochalasin B, and GLUT1 oligomeric structure are unaffected by exofacial GLUT1 proteolysis. In contrast, the cytochalasin B binding capacity of GLUT1 and the Kd(app) for cytochalasin B binding to the transporter are doubled following exofacial tryptic digestion of GLUT1. Photoaffinity labeling experiments show that increased cytochalasin B binding results from increased ligand binding to the 25 kDa carboxyl-terminal GLUT1 peptide. Proteolysis abolishes allosteric interactions between sugar import (maltose binding) and sugar export (cytochalasin B binding) sites that normally exist on adjacent subunits within the transporter complex, but interact with negative cooperativity. Following exofacial proteolysis, these sites become mutually exclusive. Dithiothreitol disrupts GLUT1 quaternary structure, inhibits 3-O-methylglucose transport, and abolishes cooperative interactions between sugar import and export sites in control cells. Studies with reconstituted purified GLUT1 confirm that the action of trypsin on cytochalasin B binding is direct, show that proteolysis increases the apparent affinity of the sugar efflux site for transported sugars, and suggest that the membrane bilayer stabilizes GLUT1 noncovalent structure and catalytic function following GLUT1 proteolysis. Collectively, these findings demonstrate that GLUT1 does not require an intact polypeptide backbone for catalytic function. They show that the multisite sugar transporter mechanism is converted to a simple ping-pong carrier mechanism following exofacial GLUT1 proteolysis. They reveal that subunit cooperativity can be lost under circumstances where cohesive structural interactions between transporter subunits are maintained. They also refute the hypothesis [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838] that rapid substrate translocation by the multisubunit erythroid glucose transporter requires cooperative interactions between subunit ligand binding sites.

Preferential association of membrane phospholipids with the human erythrocyte hexose transporter

This study reports the results of an investigation to determine to what extent the influence of membrane lipids on the human erythrocyte sugar transporter protein activity (Caruthers, A. and Melchior, D.L. (1988) Annu. Rev. Physiol. 50, 257-271) is related to lipid/protein associations in the membrane bilayer. Differential scanning calorimetry was carried out on the human erythrocyte transport protein reconstituted into artificial bilayers formed from preselected lipids. it was found that the transport protein displays a preferential and in some cases strongly preferential affinity for specific lipid types. This association is a function of lipid head group, backbone and hydrocarbon chain length. It appears that the affinity of the transport protein for various lipids can correlate with the lipid's ability to influence transporter activity. This study further suggests that certain lipids (in this case sphingomyelin) can induce an oligomeric association of HEST monomers in the bilayer.

Yeast sugar transporters

Transport of sugars is a fundamental property of all eukaryotic cells. Of particular importance is the uptake of glucose, a preferred carbon and energy source. The rate of glucose utilization in yeast is often dictated by the activity and concentration of glucose transporters in the plasma membrane. Given the importance of transport as a site of control of glycolytic flux, the regulation of glucose transporters is necessarily complex. The molecular analysis of these transporters in Saccharomyces has revealed the existence of a multigene family of sugar carriers. Recent data have raised the question of the actual role of all of these proteins in sugar catabolism, as some appear to be lowly expressed, and point mutations of these genes may confer pleiotropic phenotypes, inconsistent with a simple role as catabolic transporters. The transporters themselves appear to be intimately involved in the process of sensing glucose, a model for which there is growing support.

Alterations in red blood cell sugar transport by nanomolar concentrations of alkyl lysophospholipid

Acyl lysolipids presented in vitro to red blood cells in amounts comparable to blood serum levels inhibit protein-mediated glucose transport (Naderi, A., Carruthers, A. and Melchior, D.L. (1989) Biochim. Biophys. Acta 985, 173-181). In this study, an alkyl lysolipid (2-O-methyl-1-O-octadecyl-sn-glycero-3- phosphocholine; ALP), was found to be an order of magnitude more effective in inhibiting sugar transport than the most potent acyl lysolipid. Bilayer concentrations of ALP as low as 5 ALP molecules per transporter (0.1 mol% of total membrane lipid) result in a 50% inhibition of transport activity. ALP acts as a competitive inhibitor of exchange L-glucose transport, of CCB binding to the glucose transporter and of D-glucose inhibition of CCB binding to the transporter. Inhibition of zero-trans sugar uptake by ALP is noncompetitive. The two enantiomers of ALP show a different ability to inhibit sugar transport. The action of ALP is consistent with a mechanism in which ALP interacts with a transmembrane portion of the sugar transport molecule resulting in a competitive displacement of D-glucose or cytochalasin B from the cytosolic facing side of the transport molecule. The simplest explanation of our findings is a direct interaction of the ALP molecule with the transport protein.

The human erythrocyte sugar transporter is also a nucleotide binding protein

We have previously shown that ATP interacts with an intracellular, stereoselective, regulatory site(s) on the human erythrocyte sugar transport system to modify transport function in a hydrolysis-independent manner. This present study examines the nucleotide binding properties of the human erythrocyte sugar transport system. We demonstrate by transport studies in ghosts, by nucleotide binding studies with purified transport protein by measurements of nucleotide inhibition of 8-azidoadenosine 5'-[gamma-32P]triphosphate (azido-ATP) photoincorporation into purified carrier, and by analysis of nucleotide inhibition of carboxyl-terminal peptide antisera binding to purified glucose carrier than the glucose transport protein binds (with increasing order of affinity) AMP, ADP, ATP, 5'-adenylyl imidodiphosphate (AMP-PNP), and 1,N6-ethenoadenosine 5'-triphosphate (EATP) at a single site. The carrier lacks detectable ATPase activity and GTP binding capacity. While AMP and ADP bind to the carrier protein and act as competitive inhibitors of ATP binding, these nucleotides are unable to mimic the ability of ATP, AMP-PNP, and EATP to modify the catalytic properties of the sugar transport system. Limited tryptic digestion of azido-ATP-photolabeled carrier suggests that the region of the glucose transport protein containing the intracellular cytochalasin B binding and extracellular bis(mannose) binding domains [residues 270-456; Holman, G. D., & Rees, W. D. (1987) Biochim. Biophys. Acta 897, 395-405] may also contain the intracellular ATP binding site.

Modulation of red blood cell sugar transport by lyso-lipid

The in vitro presentation to red blood cells of specific lysolipids in amounts comparable to lysolipid levels in serum is shown to markedly influence protein-mediated glucose transport. Lysolipids were introduced exogenously into cell membranes by incubating erythrocytes in buffer containing varying concentrations of lysolipid (under 3.2 microM). The transport-modulating potency of the lysolipids was found to be dependent both on headgroup and hydrocarbon chain. MPL (monopalmitoyl lecithin, L-alpha-lysopalmitoylphosphatidylcholine) had the greatest influence on sugar transport. 15 min incubation of red cells in MPL suspensions sufficed for 99% association of the lysolipid with the cell membranes. This association correlated with altered red-cell sugar transport. At MPL/bilayer lipid molar ratios as low as 0.03%, MPL was found to act as a reversible, hyperbolic, mixed-type inhibitor of exchange D-glucose exit (both Km(app) and Vmax for transport are reduced). Dissociation of MPL from the membrane results in the recovery of original transport activity. MPL at 1.5.10(-17) mol MPL/red cell was found to reduce Ki(app) for D-glucose inhibition of cytochalasin B binding to the glucose carrier protein in red cell ghost membranes. Our findings demonstrate that red-cell membrane-exogenous lysolipid associations can significantly modify protein mediated sugar transport. The simplest explanation of our findings is a direct interaction of lysolipid with the transport protein.

Analysis of protein-mediated 3-O-methylglucose transport in rat erythrocytes: rejection of the alternating conformation carrier model for sugar transport

3-O-Methylglucose (3OMG) transport in rat erythrocytes (RBCs) is mediated by a low-capacity, facilitated diffusion-type process. This study examines whether the characteristics of sugar transport in rat RBCs are consistent with the predictions of two diametric, theoretical mechanisms for sugar transport. The one-site carrier describes a transport mechanism in which sugar influx and efflux substrate binding sites are mutually exclusive. The two-site carrier describes a transport mechanism in which sugar influx and efflux substrate binding sites can exist simultaneously but may interact in a cooperative fashion when occupied by substrate. Michaelis and velocity parameters for saturable 3OMG transport in rat erythrocytes at 24 degrees C were obtained from initial rate measurements of 3OMG transport. The results are incompatible with the predictions of the one-site carrier but are consistent with the predictions of a symmetric two-site carrier, displaying negligible cooperativity between substrate binding sites. This allows reduction of the two-site carrier transport equations to a form containing fewer constants than the one-site carrier equations without limiting their predictive success. While the available evidence does not prove that rat erythrocyte sugar transport is mediated by a two-site mechanism, we conclude that adoption of the formally more complex one-site model for sugar transport in rat erythrocytes is unnecessary and unwarranted. Counterflow experiments have also been performed in which the time course of radiolabeled 3OMG uptake is measured in cells containing saturating levels of 3OMG. The results of these experiments are consistent with the hypothesis [Naftalin et al. (1985) Biochim. Biophys. Acta 820, 235-249] that exchange of sugar between intracellular compartments (cell water and hemoglobin) can be rate limiting for transport under certain conditions.

ATP does not regulate the reconstituted glucose transporter

ATP has been reported to affect glucose transport in human erythrocytes and resealed erythrocyte ghosts [Jacquez, J. A. (1983) Biochim. Biophys. Acta 727, 367-378; Jensen, M. R., & Brahm, J. (1987) Biochim. Biophys. Acta 900, 282-290]. In more detailed studies, effects of micromolar levels of ATP on transport in ghosts and inside-out vesicles, and on the fluorescence of ghosts and the purified glucose transporter [Carruthers, A. (1986) Biochemistry 25, 3592-3602; Hebert, D. N., & Carruthers, A. (1986) J. Biol. Chem. 261, 10093-10099; Carruthers, A. (1986) J. Biol. Chem. 261, 11028-11037], have been interpreted as supporting a model in which ATP regulates the catalytic properties of the transporter. Both allosteric and covalent effects of ATP were proposed; among the allosteric effects was a 60% reduction in the Km for zero-trans uptake. In order to test whether allosteric ATP regulation of the transporter occurs, we reconstituted glucose transport activity into liposomes using erythrocyte membranes without detergent treatment. The effects of ATP, present either outside, inside, or both inside and outside the liposomes, on the transport activity were examined. Effects of ATP on trypsin-treated liposomes, which have only a single orientation of active transporters, were also tested. While the model predicts activation by ATP, only inhibition was observed. This was significant only at millimolar concentrations of ATP, in contrast to the previously reported effects at micromolar levels, and was primarily on the extracellular surface of the transporter. In addition, the ATP effects on reconstituted transport were nonspecific, with similar effects produced by tripolyphosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature dependence of glucose transport in erythrocytes from normal and alloxan-diabetic rats

Alloxan diabetes increased 3-O-methylglucose transport rates in rat red blood cells (RBC) at temperatures below 30 degrees C and decreased them above 30 degrees C. Preincubation of RBC from control rats with 20 mM glucose, 3-O-methylglucose, 2-deoxyglucose or xylose greatly elevated transport at 14 degrees C by increasing Vmax. The effect was slight at 40 degrees C. Preincubation with glucose or deoxyglucose alone caused a 50% depression of transport rates at 40 degrees C as a result of a rise in the Km, which is similar to findings in cells from alloxan-diabetic rats. Measurement of intracellular glucose metabolites suggested inhibition of glycolysis in cells from diabetic rats and a positive correlation between the level of intracellular hexose monophosphates and transport inhibition. Membrane fatty-acid and cholesterol composition and membrane lipid-ordering as monitored by electron paramagnetic resonance were not altered by alloxan diabetes. It is concluded that intracellular sugar and sugar metabolism alter the temperature dependence of glucose transport kinetics. Glucose metabolism can feed back to inhibit transport by increasing the transport Km at physiological temperatures only.

Cytochalasin inhibition of hexose transport by platelets

Previously we described a two-transporter model (T1, T2) for galactose uptake by platelets (Horne, M.K. and Hart, J.S. (1986) Biochim. Biophys. Acta 856, 448-456). In the current work we have sought corroborative evidence for this model by studying the effects of cytochalasins on this transport system. Of the various cytochalasins tested, cytochalasin B was the most potent inhibitor (I) of galactose transport, whereas cytochalasin A was less inhibitory and dihydrocytochalasin B and cytochalasin E had no inhibitory effect. The same order of potency was observed for the inhibition of L-glucose diffusion into platelets. The mechanism of cytochalasin B inhibition was investigated in detail. Inhibition of T1 was competitive and required a higher concentration of cytochalasin B (Ki1 approximately 1.7 microM) than inhibition of T2, which was of a mixed type (Ki2 approximately 0.8 microM). The effect of cytochalasin B on T2 could be accounted for by a membrane alteration which enhanced the affinity of the transporter for galactose while simultaneously preventing passage of the TSI complex into the cell. Since a similar effect on membrane permeability would also explain cytochalasin B inhibition of L-glucose diffusion, it is hypothesized that cytochalasin B binds to a membrane structure shared by T2 and the passage for L-glucose. The differences in cytochalasin B sensitivity and mechanism of inhibition manifested by T1 and T2 support our original hypothesis that galactose is indeed transported by kinetically distinct agencies and suggest that these may be physically distinct as well.

Kinetics of glucose transport in human erythrocytes: zero-trans efflux and infinite-trans efflux at 0 degree C

The kinetic features of glucose transport in human erythrocytes have been the subject of many studies, but no model is consistent with both the kinetic observations and the characteristics of the purified transporter. In order to reevaluate some of the kinetic features, initial rate measurements were performed at 0 degree C. The following kinetic parameters were obtained for fresh blood: zero-trans efflux Km = 3.4 mM, Vmax = 5.5 mM/min; infinite-trans efflux Km = 8.7 mM, Vmax = 28 mM/min. For outdated blood, somewhat different parameters were obtained: zero-trans efflux Km = 2.7 mM, Vmax = 2.4 mM/min; infinite-trans efflux Km = 19 mM, Vmax = 23 mM/min. The Km values for fresh blood differ from the previously reported values of 16 mM and 3.4 mM for zero-trans and infinite-trans efflux, respectively (Baker, G.F. and Naftalin, R.J. (1979) Biochim. Biophys. Acta 550, 474-484). The use of 50 mM galactose rather than 100 mM glucose as the infinite-trans sugar produced no change in the infinite-trans efflux Km values but somewhat lower Vmax values. Simulations indicate that initial rates were closely approximated by the experimental conditions. The observed time courses of efflux are inconsistent with a model involving rate-limiting dissociation of glucose from hemoglobin (Naftalin, R.J., Smith, P.M. and Roselaar, S.E. (1985) Biochim. Biophys. Acta 820, 235-249). The results presented here support the adequacy of the carrier model to account for the kinetics.

Evidence for non-uniform distribution of D-glucose within human red cells during net exit and counterflow

The kinetic parameters of net exit of D-glucose from human red blood cells have been measured after the cells were loaded to 18 mM, 75 mM and 120 mM at 2 degrees C and 75 mM and 120 mM at 20 degrees C. Reducing the temperature, or raising the loading concentration raises the apparent Km for net exit. Deoxygenation also reduces the Km for D-glucose exit from red blood cells loaded initially to 120 mM at 20 degrees C from 32.9 +/- 2.3 mM (13) with oxygenated blood to 20.5 +/- 1.3 mM (17) (P less than 0.01). Deoxygenation increases the ratio Vmax/Km from 5.29 +/- 0.26 min-1 (13) for oxygenated blood to 7.13 +/- 0.29 min-1 (17) for deoxygenated blood (P less than 0.001). The counterflow of D-glucose from solutions containing 1 mM 14C-labelled D-glucose was measured at 2 degrees C and 20 degrees C. Reduction in temperature, reduced the maximal level to which labelled D-glucose was accumulated and altered the course of equilibration of the specific activity of intracellular D-glucose from a single exponential to a more complex form. Raising the internal concentration from 18 mM to 90 mM at 2 degrees C also alters the course of equilibration of labelled D-glucose within the cell to a complex form. The apparent asymmetry of the transport system may be estimated from the intracellular concentrations of labelled and unlabelled sugar at the turning point of the counterflow transient. The estimates of asymmetry obtained from this approach indicate that there is no significant asymmetry at 20 degrees C and at 2 degrees C asymmetry is between 3 and 6. This is at least 20-fold less than predicted from the kinetic parameter asymmetries for net exit and entry. None of the above results fit a kinetic scheme in which the asymmetry of the transport system is controlled by intrinsic differences in the kinetic parameters at the inner and outer membrane surface. These results are consistent with a model for sugar transport in which movement between sugar within bound and free intracellular compartments can become the rate-limiting step in controlling net movement into, or out of the cell.

Reconstitution of the glucose transport activity of rat adipocytes

Rat epididymal fat cell membrane proteins were extracted from adipocyte ghosts with octylglucoside and incorporated by detergent dialysis into unilamellar phosphatidylcholine vesicles approx. 200 nm in diameter. The rate of glucose transport into the vesicles under zero-trans conditions was substrate dependent, saturable and inhibited by phloretin and cytochalasin B. Their maximum specific transport activity was 35.6 mumol/min per mg protein, and their half saturation constant for glucose was 15 mM. Glucose transport into the reconstituted vesicles was inhibited by only those sugars which competitively inhibited glucose transport into intact adipocytes. A major protein component of the vesicles was a 100 kDa protein which we had previously found to react with the affinity label maltosyl isothiocyanate (Malchoff, D.M., Olansky, L., Pohl, S. and Langdon, R.G. (1981) Fed. Proc. 40, 1893). Removal of adipocyte ghost membrane extrinsic proteins with dimethylmaleic anhydride followed by extraction of the resulting membrane pellet with octylglucoside yielded a solution which contained two major proteins, of Mr 100 000 and 85 000, with very small quantities of lower Mr proteins. Vesicles into which these proteins were incorporated had average specific transport activities of 624 mumol/min per mg protein and half saturation constants of 22 mM. Our results strongly indicate that the native glucose transporter of the rat adipocyte, like that of the human erythrocyte (Shelton, R.L. and Langdon, R.G. (1983) Biochim. Biophys. Acta 733, 25-33), is a 100 kDa protein.

Asymmetric transport of a fluorescent glucose analogue by human erythrocytes

A fluorescent glucose analogue, 6-deoxy-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-aminoglucose (NBDG), was synthesized and its interactions with the hexose transport system of the human red blood cell were investigated. NBDG entry is inhibited by increasing concentrations of D-glucose (Ki = 2 mM). However, NBDG exit is unaffected by D-glucose in red blood cells. Cytochalasin B was found to inhibit both NBDG entry and exit. NBDG accumulates in the red blood cell above the theoretical equilibrium concentration. Accumulation of NBDG is temperature-sensitive and is due to the binding of NBDG to some intracellular substance. The binding of NBDG to purified hemoglobin suggests that accumulation of NBDG by erythrocytes is due to the intracellular binding of NBDG to hemoglobin. NBDG does not accumulate in pink erythrocyte ghosts, while its rate of uptake is still inhibited by D-glucose and cytochalasin B. Although there was no apparent D-glucose inhibition of NBDG exit by intact red blood cells, D-glucose was able to inhibit NBDG exit by pink erythrocyte ghosts. The differing properties of NBDG influx and efflux support the interpretation that the hexose transport system of the human red blood cell appears asymmetric although it may be intrinsically symmetric.

A quenched-flow technique for the measurement of glucose influx into human red blood cells

A quenched-flow apparatus is described and applied to measurements of the hydrolysis of 2,4-dinitrophenyl acetate by sodium hydroxide and the entry of D-[U-14C]glucose into human red blood cells at 37 degrees C. Glucose influx into red cells was a saturable process obeying Michaelis-Menten kinetics with a Km for glucose of 6.6 +/- 0.61 mM and a maximum rate for glucose entry under "zero trans" conditions of 20.7 +/- 0.76 mmol (L cell water)-1 s-1. The technique used requires only readily available laboratory equipment and should be easily adaptable to the study of other rapid transport processes.

Na+-coupled glycine transport in reticulocyte vesicles of distinct sidedness: stoichiometry and symmetry

Sodium-coupled glycine transport has been studied using membrane vesicles of distinct sidedness, either inside-out or right side-out, prepared from sheep reticulocytes. The activity is chloride dependent and characterized by high and low apparent affinities for glycine (K'm approximately equal to 0.5 mM and greater than 10 mM) for both types of vesicles as well as intact cells. Transport is symmetrical with respect to similar apparent affinity constants for glycine, for both the high- and low-affinity systems, and for sodium. Direct measurements of the sodium/glycine coupling indicate a ratio of 2:1, consistent with kinetic data fitted to a Hill-type equation describing glycine flux as a function of sodium concentration.